Weak/Distorted Sound from old Roland Cube 40 (Orange)

[jeffects]

I have one of the old orange Roland Cube 40s that I was initially working on to solve the problem of no reverb. Turns out someone had replaced the reverb driver chip and installed it backwards, so I fixed that. Everything worked normally on the bench so I reassembled the amp to test it with a guitar. It played well for about a minute and then the volume dropped and the tone became very distorted. The sound I'm hearing is very similar to a pedal that has a dying battery. If you play very softly the signal is fairly normal but faint, but if you strum loudly the amp crackles and breaks up. I feel this is something obvious that I'm overlooking so I could really use some help.

Here is what I've diagnosed so far:

• I've scoped the board and the preamp section is fine.
• All the transistors in the main section test okay.
• The voltage rails to the main section are what they should be +35V and -35V.
• All electrolytic caps have been replaced.

This pretty much leaves resistors and non-polarized low-capacitance caps. What does this sound like to you guys? Thanks.

[MarkusBass]

A what happens if you provide signal to the MAIN IN (power amp) input? Or, you connect PRE OUT (preamp output) to another amp?
It can be anything from a broken wire, or a cap installed backwards. You need to provide some more information.

Mark

[jeffects]

Same problem when you feed signal into the MAIN IN. PRE OUT is clean. Immediately after the MAIN IN is a .1 cap, then a 47k resistor to ground, and finally a 1k resistor that feeds into the first transistor. Probing on either side of the 1k resistor is the last place in the chain that I can get a clean signal.

[MarkusBass]

Shortly speaking the power amp has failed. Have you check power transistors? What happens when you disconnect power transistors and scope the power amp output with e.g. 330 Ohms resistor on the output instead of the speaker? Is speaker OK?

Mark

[jeffects]

Power transistors are good. I will disconnect them and scope the output to see if the signal changes so thank you for the good idea. I will also check the speaker and let you know. Thank you.

[jeffects]

Hold the phone. Q10, which is a driver transistor is bad so I was wrong in my earlier statement about all the transistors testing okay. Got a replacement on order and we'll see what happens.